City of Sammamish, WA Weather

Synopsis

Troughing will persist over the Pacific Northwest into Tuesday for unsettled conditions and periodic showers. A frontal system on Wednesday brings increasing steadier rainfall and mountain snow. An upper low then lingers over Western Washington into late week for unsettled weather and cooler temperatures.

Short Term

The weather system that was offshore continues to move inland this evening with a cold front draped across Puget Sound. Showers will continue at times into Tuesday with troughing persistent over the Pacific Northwest. High temperatures range in the mid to upper 50s on Tuesday.

Another frontal system moves southwards from British Columbia Tuesday evening through Wednesday, resulting in the return of steadier rainfall for the lowlands, especially Tuesday night. Southwest winds also increase for portions of southern and central Puget Sound late Tuesday, peaking between 20 to 30 MPH from Seattle to Tacoma and westwards towards Shelton. The most impactful aspect to this system will be the first widespread mountain snow event for the Cascades and Olympics this season. Snow levels initially around 4000 to 4500 feet fall to 3000 to 3500 feet by Wednesday morning, resulting in snow for the higher elevation Cascade Passes. At this time, the heaviest snowfall is forecast for White Pass and Paradise (6 to 12 inches, with localized amounts to 15 inches around Paradise). Snowfall accumulations of 4 to 8 inches are anticipated for Stevens and Rainy Pass, with up to 3 inches at Snoqualmie Pass due to slightly higher snow levels above 3000 feet. For this reason, a Winter Weather Advisory has been issued for the Cascades, those traveling will want to be aware of the incoming snow, and check WSDOT for updates.

Showers then continue through Wednesday night as the weather system slowly moves southwards along the offshore waters. Temperatures will be slightly cooler Wednesday with highs only in the low to mid 50s.

Long Term

Ensembles are consistent in the upper low lingering over Western Washington through at least Friday, which results in continued unsettled conditions with showers at times. Snow levels fluctuate between 2500 to 4000 feet Thursday and Friday, with light snow accumulations at times for the Cascades and Olympics. Uncertainty increases next weekend in regards to how quickly the upper trough exits the Pacific Northwest. At this time, the trend is for a slow drying trend over the weekend into early next week. Temperatures will become cooler over the weekend, and will need to monitor the threat for frost and freeze conditions Saturday into early next week. JD

Aviation

Flow aloft will transition to northwesterly as a low continues to push inland across northern Oregon. Surface winds remain light and variable this evening at generally 5 kt or less. Expect a gradual transition to the southwest for the majority of the interior terminals.

Scattered shower activity continues across the area this evening. Conditions are mainly VFR at the majority of the area TAF sites though with ample low level moisture and a switch back to onshore flow, expect MVFR, to localized IFR, conditions to develop tonight into Tuesday.

KSEA...VFR conditions are expected to persist this evening, with SW winds between 3-6 kt. Conditions will drop back down to MVFR overnight into early Tuesday, with latest probabilistic guidance hinting at roughly a 15% chance of conditions becoming IFR by 12Z. SW wind will increase to 7-12 kt on Tuesday afternoon.

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